Wireless appliances, such as wireless LANs complying with IEEE802.11a/b/g standards, and Bluetooth, have been proliferated in recent years. IEEE 802.11a and IEEE 802.11g specified the data transmission rate of 54 Mbps, and recently, active researches and developments have been done on wireless schemes for achieving higher transmission rates.
As one of techniques for increasing transmission rates of wireless communication systems, a MIMO (Multi-Input Multi-Output) communication system has received wide attention. This is a technique for increasing transmission capacity and improving communication speed by providing each of a transmitter and a receiver with multiple antenna elements and having transmission paths spatially multiplexed. This technique is essential not only for wireless LANs, but also for next-generation wireless communication systems such as mobile phone communication systems and IEEE 802.16e (WiMAX).
According to the MIMO communication scheme, the transmitter divides and sends transmitting data through the multiple active antenna elements, the data is transmitted over multiple virtual MIMO channels, and the receiver receives signals through the multiple antenna elements and processes the signals to obtain received data. In general, a wireless communication apparatus using the MIMO communication scheme is provided with multiple omnidirectional active antenna elements such as dipole antennas or sleeve antennas. In this case, there is a problem of degradation in transmission quality caused by increases in the correlations between active antenna elements, unless addressing this situation by, e.g., sufficiently separating the antenna elements from one another, or tilting the respective antenna elements in different directions to make a combination of different polarizations.
Among prior arts available for solving the above problem, for example, an array antenna apparatus disclosed in Patent Literature 1, which is an adaptive directional antenna, may be used. The array antenna apparatus disclosed in Patent Literature 1 is configured such that a half-wave dipole antenna is mounted perpendicularly on a dielectric support substrate, and three printed wiring boards are disposed to surround the half-wave dipole antenna. The half-wave dipole antenna is supplied with a radio frequency signal through a balanced feeder cable. Moreover, on the back side of each printed wiring board, two sets of passive antenna elements (parasitic elements) are disposed in parallel with each other, each set including two printed antenna elements (elements each made of a conductor pattern). In each parasitic element, the two printed antenna elements oppose to each other with a space therebetween. A through-hole conductor is provided at one end of each printed antenna element opposing to the other printed antenna element, and is connected to an electrode terminal on the front side of the printed wiring board. In each parasitic element, a variable-capacitance diode is mounted between the two electrode terminals, these electrode terminals are further connected to a pair of cables through high value resistors for blocking radio frequencies, and the pair of cables are connected to bias voltage supply terminals DC+ and DC− of a controller (not shown) for controlling to steer the array antenna apparatus. By changing bias voltages supplied from the controller, the respective reactance values of the variable-capacitance diodes connected to the parasitic elements change. In this manner, the electrical length of each parasitic element is changed as compared to that of the half-wave dipole antenna, thus changing the horizontal radiation pattern of the array antenna apparatus.
Moreover, an antenna apparatus disclosed in Patent Literature 2 is configured to include: a linear radiating element disposed on a first surface; a first parasitic element disposed on the first surface and parallel to the radiating element; a first grounding conductor disposed on the first surface; first switches for connecting both ends of the first parasitic element to the first grounding conductor; a second grounding conductor disposed on a second surface opposite to the first surface; and control means for controlling the close and open of the switches. A part of the first grounding conductor is disposed parallel to the radiating element, and opposite to the first parasitic element with respect to the radiating element and. The second grounding conductor is opposed to the radiating element, and an edge of the second grounding conductor is opposed to a region between the radiating element and the first parasitic element. According to the antenna apparatus of this invention and a wireless terminal using the antenna apparatus, the antenna directivity can be changed between back and zenith directions by closing or opening the switches. Thus, even when the wireless terminal has different usage modes, such as a voice call mode and a data communication mode, it is possible to perform high-quality communication by changing the antenna directivity to the one suitable for a particular usage mode.
In the case of performing MIMO communication, it is possible to use an array antenna apparatus including a plurality of steerable antennas disclosed in Patent Literature 1 or 2, and thus, to set each steerable antenna's radiation pattern so as to reduce the correlations among active antenna elements.